Unconventional hydrocarbons production from low-permeability organic-rich shale and tight-sand formations is rapidly expanding. It opens vast new energy sources to the nation. Gas productions in 2012 from shale and tight-sand formations were, respectively, accounting for 34% and 24% of the nation's total gas production. Unconventional gas production is currently dominant in Marcellus Basin (29%), Haynesville Basin (23%), Barnett Basin (17%), and the remaining 31% is contributed by about 20 other basins. Future forecasts indicate that unconventional gas production will double by 2035 whereas unconventional oil production will increase by about 15%.
However, unconventional hydrocarbons exploration may impose adverse health and environmental long-term effects. In shale and tight-sand exploration, a fluid is used to fracture and stimulate the formation. This fluid is referred to as fracturing fluid or completion fluid. Fracturing fluid is typically potable water mixed with a large number of additives, and some of which are toxic chemicals. The average required volume of fracturing fluid is about 40,000 barrels for a vertical well and 100,000 barrels for a horizontal well.
The fluid that flows back during and after fracturing is often denoted flow-back water (FBW) or produced water (PW). However, it is referred to in this invention as PW. A portion of the fracturing fluid (e.g., 20-45%) flows back to the surface as PW, and the flow of PW substantially decreases with time to near halation at the well completion (30-90 days). During fracturing, ions including alkaline cations (magnesium, calcium, strontium, barium and radium), Naturally Occurring Radioactive Materials (NORM; the decay series of radium), and transition metals within formation layers are dissolved, mixed with high salinity formation water, and mobilized to the surface with PW. As such, ions concentrations in PW sharply increase with time; the longer the downhole residence time, the higher their concentrations in PW. Table 1 [Haluszczak, L. O., et al., Applied Geochemistry, 2013, 28, pp. 55-61] and Table 2 [Hayes, T. and Severin, B. F., RPSEA Final Report #08122-05, 2012], for example, present some samples of PW at 10-14 days from, respectively, Marcellus and Barnett basins. Alkaline cations (magnesium, calcium, strontium, barium and radium), particularly radium radioactive isotopes, along with bromide are signatures for such PW.
Hydro-fracturing is thus faced with surmounted predicaments driven by negative public perception for several reasons. First, the 2005 Energy Policy Act exempts fracturing operations from the Safe Drinking Water Act (SDWA) with one exception (injection of diesel) to the exemption. Health and environmental regulations, monitoring, and enforcement for various contaminants by individual states were moot. Such, due to public pressure, are now continually evolving, which focus on the disclosure of the constituents in fracturing fluids as well as in PW (discharge and reuse of PW). One of the critical moves is the development of appropriate analytical methods, particularly for radioactivity, to replace the “drift” of extending existing analytical procedures, which may not be applicable (e.g., outdated) to such PW and tend to underestimate the levels of some critical contaminants.
Second, the toxic-nature of fracturing fluids as well as the toxic- and possible radioactive-nature of PW are alarming. The journey of fracturing fluids from the surface to downhole formation and back to the surface with carried over PW may contaminate groundwater aquifers via geological connectivity and leakage. Also, of concern is leakage from PW holding ponds.
Third, the common method to manage PW is of course disposal in deep wells. However, the demand for such disposal wells is overwhelming due to the sheer volume of PW. For example, PW from the Marcellus basin was over 31 million barrels in 2014 alone. In addition, the potential risks of contaminating potable water aquifers (e.g., leakage of disposal wells and geological connectivity) and inducing seismicity and earthquakes are high. As such, disposal wells may be limited or not available for producers in some states (e.g., Pennsylvania). The alternative of a long distance hauling and disposing of PW in deep wells in neighboring states is also relatively expensive. For example, the combined cost of trucking PW from Pennsylvania to a neighboring state and disposing PW in deep wells in the neighboring state may be $10-15 per barrel.
The staggering volume of PW indicates the least the dependency of hydro-fracturing on potable water, if not overuse or depletion of potable water resources, where hydro-fracturing may compete with other uses especially in water distressed areas. Yet, direct disposal of PW in deep wells is becoming not easily accessible, relatively expensive and very risky. Yet, the predominant theme so far in treating PW is dilution, but dilution is not a solution to pollution. Pseudo treatment of PW, especially when the content of PW, whether it's in a liquid or a solid form, is released into groundwater, surface water, domestic wastewater treatment plants, public roads (e.g., via roads de-icing salts) and landfills, spreads contaminants wider and further even if the release is within regulations. Such critical issues open the door for promoting and developing innovative methods to properly treat and reuse PW.